1. Field of the Invention
The present invention relates to an auto focus apparatus, and more particularly, to an auto focus apparatus of a camera using plastic lenses for a photographing optical system.
2. Description of the Related Art
A plastic lens can be easily formed using a metal die, has a high degree of flexibility in the shape and can be manufactured at lower cost than a glass lens, and therefore an increasing number of cameras use plastic lens in a photographing optical system.
However, since plastic has a larger thermal expansion coefficient with respect to a temperature variation than glass and a disadvantage of plastic is that when used for a lens, an image forming position may vary depending on an environment in which the lens is used.
Therefore, the range (in-focus state search range) of driving a focus lens of a camera using plastic lenses is often set with a certain margin to allow focusing even when an in-focus position changes due to a temperature variation.
However, setting the range of driving a focus lens with a certain margin may cause the focus lens to be driven up to a point at which there could be originally no in-focus position, with the result that focusing takes a longer time.
Therefore, it is a conventional practice that the temperature of an optical system is detected by a temperature detection device, a temperature variation from a reference temperature is calculated, an amount of deviation of the in-focus position due to the temperature variation is calculated, and the focus driving range at the reference temperature is corrected so as to shorten the time required for focusing (e.g., Japanese Patent Application Publication No. 11-142714).
Furthermore, an amount of correction of the focus driving section with respect to a temperature variation around the lens is stored in a storage section beforehand and when a temperature variation occurs, the focus driving section is controlled based on information on the amount of correction stored in the storage section to prevent deviation of the in-focus position due to the temperature variation (e.g., Japanese Patent Application Publication No. 2003-131106).